This is a nice surprise.
My only knowledge with Environments came from Germán's work in the GSoC 2010, as demonstrated here:
http://www.youtube.com/watch?v=n4I7fSVNX2A
Your work is quite different. No Environments added as classes in a browser. I remember importing code into an Environment being a showstopper. You've imported Seaside.
I guess you could have thirty Environments with thirty people doing the HelloWorld demo with #renderContentOn:. One image serving thirty people over the wire.
I imagine that the main concern is collisions and not security. But I guess if you're importing Smalltalk globals into Environment>>lookup, then you can mask out the classes you don't want an Environment to be able to access. Say, remove FileDirectory and FileStream, so that those hypothetical Seaside users couldn't explore the file system and send the results back to the browser.
Cool stuff.
Chris